Impact of the non-antibiotic compound vitamin C on ciprofloxacin efficacy: An in vitro study

Abstract Objective : Antimicrobial resistance has become a worldwide health challenge due to antibiotic misuse; thus, there is a rising interest in repurposing non-antibiotic substances, such as vitamin C. Whether these compounds can alter antibiotic efficacy remains insufficiently investigated, especially alongside commonly used antibiotics like ciprofloxacin. This study aims to evaluate the impact of vitamin C on ciprofloxacin activity in standard bacterial strains. Methods : Ciprofloxacin and vitamin C were assessed by checkerboard assay on six ATCC strains: methicillin-susceptible Staphylococcus aureus , methicillin-resistant Staphylococcus aureus, Enterococcus faecalis, Pseudomonas aeruginosa, Escherichia coli , and Klebsiella pneumoniae . The research was conducted in triplicate to ascertain minimum inhibitory concentrations and calculate the fractional inhibitory concentration index (FICI). Data were summarized with means and standard deviations, classified by outcome, and analyzed with Fisher’s exact test. Figures were created using R software. Results : For both methicillin-susceptible and methicillin-resistant Staphylococcus aureus, Enterococcus faecalis , and Klebsiella pneumoniae , all combinations showed indifference (FICI range 0.83-4), while Pseudomonas aeruginosa showed one antagonistic outcome (FICI=5). In Escherichia coli , antagonism was predominant (n=9, FICI range 4.001-6), with a statistically significant reduction in complete inhibition compared to ciprofloxacin alone (p=0.037), suggesting that vitamin C reduces ciprofloxacin efficacy at higher concentrations, while synergy occurred at lower concentrations (n=4, FICI range 0.064-0.281). Conclusions : To our knowledge, this is the first systematic checkerboard analysis of ciprofloxacin-vitamin C on multiple ATCC strains, underscoring the impact of non-antibiotic compounds. These findings are significant because they support the need for further studies on how non-antibiotic compounds may influence antibiotic therapy in patients.